changing the "alt" tag of images

Hi,

could someone explain me how to change the content of the alt tag of an image ? Currently, I have the filename in this tag and would like to put the IPTC image title or description in it. When i look the source code of my generated page i get : " <img src="/gallery2/d/24-3/Baie+de+Somme_01.jpg" width="600" height="400" alt="Baie de Somme_01"/> "

Many thanks by advance